1. Field of the Invention
The present invention relates to a composition for forming Ba.sub.1-x Sr.sub.x Ti.sub.y O.sub.3 thin films and a method for forming Ba.sub.1-x Sr.sub.x Ti.sub.y O.sub.3 thin films, and more particularly to a composition and method for forming dielectric Ba.sub.1-x Sr.sub.x Ti.sub.y O.sub.3 thin films which are effectively used to form thin-film condensers or capacitors.
2. Discussion of the Background
Since dielectric Ba.sub.1-x Sr.sub.x Ti.sub.y O.sub.3 thin films have high dielectric constants, attention has recently been focused on their use for formation of capacitors for semiconductor memories, instead of SiO.sub.2 or SiN.sub.x, or built-in condensers for signal processing IC's.
Known attempted methods for formation of such thin films include CVD, sputtering, etc., drawbacks to all of which are complicated equipment and slow rates of formation of thin films. They also have the problems of leaving only small areas for forming thin films, and thus fail to provide films with large areas.
In contrast, the sol-gel method which uses a liquid raw material has advantages in that the composition can be readily controlled through the use of a variety of substituent elements or additives, and thin films with large areas can be obtained at low cost through a relatively simple process, and is therefore an industrially promising method.
The sol-gel method is a process which involves mixing metal salts or metal alkoxides as raw materials for Ba, Sr and Ti, into an organic solvent, and coating a substrate with the resulting solution, followed by crystallization. In cases where thin films are formed according to this method, since the metal salts or the metal alkoxides exhibit higher degrees of solubility in the organic solvent, after the coating solution has been coated, the films are usually dried at room temperature and 150.degree. C., and then undergo calcination at 500.degree.-600.degree. C. for 1 hour, or at 750.degree. C. or a higher temperature for 1 minute. In order to increase the film thickness, the steps of coating, drying and calcination are repeated, and finally firing at a firing temperature of 650.degree. C. or higher is effected for crystallization.
Here, the raw materials for the coating solution can be metal alkoxides for any of Ba, Sr and Ti (M. N. Kamalasanan, et al., J. Appl. Phys. 74(9):0021-8979, 1993); or carboxylic salts of Ba and Sr, and an alkoxide of Ti (H. K. Chae, et al., Mat. Res. Soc. Symp. Proc. Vol. 271, 1992).
The recent need for increased densities and multifunctionality of electronic circuits requires three-dimensional, multi-layer wiring on substrates and simplified processes for their preparation. According to the conventional processes for forming thin films, however, recoating must be repeatedly followed by the step of calcination at a high temperature, such as 500.degree.-600.degree. C., and the firing temperature for crystallization is high as well. Therefore it has been difficult to meet the requirements as mentioned above, due to possible impairment of transistor devices, etc. or changes in properties which result from production of unwanted oxides.
On the other hand, since the use of a carboxylic salt as one of the raw materials allows sintering with a relatively small quantity of heat and with a low shrinkage factor which results in the advantage of resistance to cracking of the thin films formed, the following compositions for forming thin films have been proposed as well.
(i) A composition for forming barium titanate thin films, which is prepared by dissolving a barium salt of a lower-carboxylic acid (preferably, barium acetate) and titanium isopropoxide in an organic solvent which contains ethylene glycol monomethyl ether, and contacting the resulting solution with water (Japanese Unexamined Patent Application Disclosure HEI 1-100024); and PA1 (ii) a composition for forming barium titanate thin films, which is prepared by dissolving barium 2-ethylhexanoate and titanium isopropoxide in methanol as the solvent which contains acetylacetone (J. Appl. Phys. 74(9):1, November 1993). PA1 (iii) A method for forming SrTiO.sub.3 films, which comprises drying isopropoxy strontium and isopropoxy titanium as the raw materials at 150.degree. C., followed by a first firing at 620.degree. C. and a second firing at 700.degree. C. for crystallization (the symposium summary for the 1992 symposium held by The Ceramic Society of Japan, p. 4); PA1 (iv) a method for forming BaTiO.sub.3 films by crystallizing barium acetate and n-butoxy titanium as the raw materials at 700.degree.-900.degree. C. (J. of The Ceramic Society of Japan, 98(8):743-748, 1990); and PA1 (v) a method for forming BaTiO.sub.3 films by crystallizing metal soaps of barium and titanium at 600.degree.-1300.degree. C. (Japanese Unexamined Patent Application Disclosure HEI 1-308801). PA1 (A) R.sup.a COOH wherein R.sup.a is a linear or branched alkyl group having 3-7 carbon atoms; and PA1 (B) Ti(OR.sup.b).sub.4 wherein R.sup.b is a linear or branched alkyl group having 1-7 carbon atoms. PA1 those with R.sup.a having 3 carbon atoms: n-butyric acid and isobutyric acid; PA1 those with R.sup.a having 4 carbon atoms: n-valeric acid and isovaleric acid; PA1 those with R.sup.a having 5 carbon atoms: 2-ethylbutyric acid, n-hexanoic acid, 2,2-dimethylbutyric acid, 3,3-dimethylbutyric acid and 4-methylpentanoic acid; PA1 those with R.sup.a having 6 carbon atoms: n-heptanoic acid and 2-methylhexanoic acid; and PA1 those with R.sup.a having 7 carbon atoms: octylic acid.
For preparation of the conventional compositions (i) and (ii) above for forming thin films, additives such as ethylene glycol monomethyl ether, acetylacetone, etc. are added in order to increase the solubility of the carboxylate which is barely soluble in the solvent, and to improve the homogeneity of the thin films. The addition of such additives, however, causes an increase in the decomposition temperature of the resulting metal complex, and this results in the problem of an increased firing temperature.
Practical firing temperatures are all relatively high as evidenced by the firing temperature described in Japanese Unexamined Patent Application Disclosure HEI 1-100024, for example, 800.degree.-1300.degree., or 750.degree. C., the firing temperature described in J. Appl. Phys. 74(9):1, November 1993. The conventional compositions which require these high firing temperatures have presented problems; the underlying substrate is prone to degradation due to the high heat during the firing, and the electric properties of the resulting thin films, including their dielectric constants, tend to be impaired.
In addition, the following methods have been presented for the formation of thin films as well.
However, in (iii) alkoxides are used as the raw materials for Ba and Sr. CO.sub.2, which is evolved during the first firing, reacts with the Ba and Sr already present, as the result of hydrolysis, to produce BaCO.sub.3 and SrCO.sub.3 which then react with TiO.sub.2 to form BaTiO.sub.3, SrTiO.sub.3 or Ba.sub.1-x Sr.sub.x TiO.sub.3 while releasing CO.sub.2. Accordingly, the amount varies greatly due to the release of CO.sub.2, resulting in increased shrinkage of the films which tends to cause cracking. Likewise, in (iv) above wherein barium acetate is used as one of the raw materials, a carbonic salt tends to be formed, and thus cracking easily occurs for the same reason as mentioned above. On the other hand, when metallic soaps of carboxylic acids having many carbon atoms are used according to (v) above, although no carbonic salts are formed during the heat decomposition, the amount varies greatly due to evaporation of the organic components, and eventually, cracking still tends to occur.
Even with a titanium alkoxide, the amount varies greatly due to evaporation and decomposition of the organic component during the firing, since the hydrolysis fails during formation of the films as the length of the carbon chain increases, thus resulting in cracking.